This invention relates to underfill encapsulant compositions prepared from allylated amide compounds to protect and reinforce the interconnections between an electronic component and a substrate in a microelectronic device.
Microelectronic devices contain millions of electrical circuit components, mainly transistors assembled in integrated circuit (IC) chips, but also resistors, capacitors, and other components. These electronic components are interconnected to form the circuits, and eventually are connected to and supported on a carrier or substrate, such as a printed wire board.
The integrated circuit component may comprise a single bare chip, a single encapsulated chip, or an encapsulated package of multiple chips. The single bare chip can be attached to a lead frame, which in turn is encapsulated and attached to the printed wire board, or it can be directly attached to the printed wire board.
Whether the component is a bare chip connected to a lead frame, or a package connected to a printed wire board or other substrate, the connections are made between electrical terminations on the electronic component and corresponding electrical terminations on the substrate. One method for making these connections uses metallic or polymeric material that is applied in bumps to the component or substrate terminals. The terminals are aligned and contacted together and the resulting assembly heated to reflow the metallic or polymeric material and solidify the connection.
During subsequent manufacturing steps, the electronic assembly is subjected to cycles of elevated and lowered temperatures. Due to the differences in the coefficient of thermal expansion for the electronic component, the interconnect material, and the substrate, this thermal cycling can stress the components of the assembly and cause it to fail. To prevent failure, the gap between the component and the substrate is filled with a polymeric encapsulant, hereinafter called underfill or underfill encapsulant, to reinforce the interconnect and to absorb some of the stress of the thermal cycling.
Two prominent uses for underfill technology are in packages known in the industry as flip-chip, in which a chip is attached to a lead frame, and ball grid array, in which a package of one or more chips is attached to a printed wire board.
The underfill encapsulation may take place after the reflow of the metallic or polymeric interconnect, or it may take place simultaneously with the reflow. If underfill encapsulation takes place after reflow of the interconnect, a measured amount of underfill encapsulant material will be dispensed along one or more peripheral sides of the electronic assembly and capillary action within the component-to-substrate gap draws the material inward. The substrate may be preheated if needed to achieve the desired level of encapsulant viscosity for the optimum capillary action. After the gap is filled, additional underfill encapsulant may be dispensed along the complete assembly periphery to help reduce stress concentrations and prolong the fatigue life of the assembled structure. The underfill encapsulant is subsequently cured to reach its optimized final properties.
If underfill encapsulation is to take place simultaneously with reflow of the solder or polymeric interconnects, the underfill encapsulant, which can include a fluxing agent if solder is the interconnect material, first is applied to either the substrate or the component; then terminals on the component and substrate are aligned and contacted and the assembly heated to reflow the metallic or polymeric interconnect material. During this heating process, curing of the underrill encapsulant occurs simultaneously with reflow of the metallic or polymeric interconnect material.
For single chip packaging involving high volume commodity products, a failed chip can be discarded without significant loss. However, it becomes expensive to discard multi-chip packages with only one failed chip and the ability to rework the failed component would be a manufacturing advantage. Today, one of the primary thrusts within the semiconductor industry is to develop not only an underfill encapsulant that will meet all the requirements for reinforcement of the interconnect, but also an underfill encapsulant that will be reworkable, allowing for the failed component to be removed without destroying the substrate.
Conventional underfill technology uses low viscosity thermosetting organic materials, the most widely used being epoxy/anhydride systems. In order to achieve the required mechanical performance, relatively high molecular weight thermoplastics would be the preferred compositions for underfill materials. These materials, however, have high viscosity or even solid film form, which are drawbacks to the manufacturing process. Therefore, there is a need for new underfill encapsulant compositions that are easily dispensable to conform with automated manufacturing processes, and that are reworkable.
This invention relates to a curable underfill encapsulant compositions comprising allylated amide compounds, a free radical curing agent and/or a photoinitiator, and optionally, one or more fillers or other additives. The composition optionally may also contain mono- or polyfunctional vinyl compounds.
The composition can be designed to be reworkable by choosing a major amount of mono-functional compounds for the composition.
The ability to process these compositions for underfill encapsulants is achieved by using relatively low molecular weight reactive oligomers or prepolymers and curing them in situ after application to the electronic assembly. The relatively low molecular weight translates to a lower viscosity and ease of application to the substrate.
In another embodiment, this invention is a cured encapsulant composition that results after the curing of the just described curable underfill encapsulant composition.
In another embodiment, this invention is a microelectronic assembly comprising an electronic component having a plurality of electrical terminations, each termination electrically and mechanically connected by a metallic or polymeric material (the metallic or polymeric material also referred to herein as interconnect or interconnect material) to a substrate having a plurality of electrical terminations corresponding to the terminations of the electrical component, and a cured encapsulant disposed between the electrical component and the substrate to reinforce the solder or polymeric interconnects, in which the cured encapsulant was prepared from curing a composition comprising one or more allylated amide compounds, a free radical curing agent and/or a photoinitiator, and optionally, one or more fillers. The composition optionally may also contain mono- or polyfunctional vinyl compounds.
In another embodiment this invention is a method for making an electronic assembly, the electronic assembly comprising an electronic component having a plurality of electrical terminations, each termination electrically and mechanically connected by a metallic or polymeric material to a substrate having a plurality of electrical terminations corresponding to the terminations of the electrical component, and a cured reworkable underfill encapsulant composition disposed between the electronic component and the substrate, the method comprising: (a) providing a curable underfill encapsulant composition, (b) disposing the curable composition between the electrical component and the substrate; and (c) curing the composition in situ.
The allylated amide, and vinyl compounds, used in the underfill encapsulant compositions of this invention are curable compounds, meaning that they are capable of polymerization, with or without crosslinking. As used in this specification, to cure will mean to polymerize, with or without crosslinking. Cross-linking, as is understood in the art, is the attachment of two polymer chains by bridges of an element, a molecular group, or a compound, and in general will take place upon heating. As cross-linking density is increased, the properties of a material can be changed from thermoplastic to thermosetting, which consequently increases polymeric strength, heat-and electrical resistance, and resistance to solvents and other chemicals.
It is possible to prepare polymers of a wide range of cross-link density, from tacky, elastomeric to tough glassy polymers, by the judicious choice and amount of mono- or polyfunctional compounds. The greater proportion of polyfunctional compounds reacted, the greater the cross-link density. If thermoplastic properties are desired, the underfill encapsulants of this invention can be prepared from mono-functional compounds to limit the cross-link density. However, a minor amount of poly-functional compounds can be added to provide some cross-linking and strength to the composition, provided the amount of poly-functional compounds is limited to an amount that does not diminish the desired thermoplastic properties. Within these parameters, the strength and elasticity of individual underfill encapsulants can be tailored to a particular end-use application.
The cross-link density can also be controlled to give a wide range of glass transition temperatures in the cured underfill to withstand subsequent processing and operation temperatures.
For those underfill encapsulants that are designed to be reworkable, the Tg is chosen to be below the reflow temperature of the metallic or polymeric interconnect. If the underfill is added after the reflow of the interconnect material, the low Tg will allow the encapsulant material to soften and adhere to both the electronic component and the substrate without affecting the interconnect.
If the underfill is added before the reflow, the same effect is achieved. The underfill encapsulant will soften and adhere to the electronic component and substrate during the reflow of the interconnect material. Intimate contact is maintained with the interconnect after the interconnect solidifies, imparting good stress transfer and long-term reliability.
In those cases where it is necessary to rework the assembly, the electronic component can be pried off the substrate, and any residue underfill can be heated until it softens and is easily removed.
In the inventive underfill encapsulant compositions, the allylated amide compounds, and vinyl compounds if used in combination with the allylated amide compounds, will be present in the curable underrill encapsulant compositions in an amount from 2 to 98 weight percent based on the organic components present (excluding any fillers).
The underfill encapsulant compositions will further comprise at least one free-radical initiator, which is defined to be a chemical species that decomposes to a molecular fragment having one or more unpaired electrons, highly reactive and usually short-lived, which is capable of initiating a chemical reaction by means of a chain mechanism. The free-radical initiator will be present in an amount of 0.1 to 10 percent, preferably 0.1 to 3.0 percent, by weight of the organic compounds present, excluding any filler. The free radical curing mechanism gives a fast cure and provides the composition with a long shelf life before cure. Preferred free-radical initiators include peroxides, such as butyl peroctoates and dicumyl peroxide, and azo compounds, such as 2,2xe2x80x2-azobis(2-methyl-propanenitrile) and 2,2xe2x80x2-azobis(2-methyl-butanenitrile).
Alternatively, the underfill encapsulant compositions may contain a photoinitiator in lieu of the free-radical initiator, and the curing process may then be initiated by UV radiation. The photoinitiator will be present in an amount of 0.1 to 10 percent, preferably 0.1 to 3.0 percent, by weight of the allylated amide compound, or combination of both allylated amide and vinyl compounds, present (excluding any filler). In some cases, both photoinitiation and free-radical initiation may be desirable. For example, the curing process can be started by UV irradiation, and in a later processing step, curing can be completed by the application of heat to accomplish a free-radical cure.
In general, these compositions will cure within a temperature range of 50xc2x0 to 250xc2x0 C., and curing will be effected within a length of time of less than one minute to four hours. As will be understood, the time and temperature curing profile for each adhesive composition will vary, and different compositions can be designed to provide the curing profile that will be suited to the particular industrial manufacturing process.
Ease of application, even when thermoplastic properties are desired for the underfill encapsulant, is achieved by using relatively low molecular weight reactive oligomers or pre-polymers and curing these in situ after application to the electronic assembly of component and substrate. Applying the materials in an uncured state gives high processibility, and the resultant cured thermoplastic encapsulant provides high mechanical performance.
For some underfill operations, inert inorganic fillers are used in the underfill encapsulant to adjust the coefficient of thermal expansion to more closely mirror that of the circuit interconnect, and to mechanically reinforce the interconnect. Examples of suitable thermally conductive fillers include silica, graphite, aluminum nitride, silicon carbide, boron nitride, diamond dust, and clays. The fillers will be present typically in an amount of 20-80 percent by weight of the total underrill encapsulant composition.
The allylated amide compounds suitable for use in the compositions of this invention have a structure represented by the formulas A and B as depicted here: 
As used throughout this specification, the notation C(O) refers to a carbonyl group. For these specific formulae, when lower case xe2x80x9cnxe2x80x9d is the integer 1, the compound will be a mono-functional compound; and when lower case xe2x80x9cnxe2x80x9d is an integer 2 to 6, the compound will be a poly-functional compound.
Formula A represents those compounds in which:
R9 is H, an alkyl or alkyleneoxy group having 1 to 18 carbon atoms, allyl, aryl, or substituted aryl having the structure 
xe2x80x83in which R10, R11, and R12 are independently H or an alkyl or alkyleneoxy group having 1 to 18 carbon atoms;
each Ar independently is an aromatic group selected from the aromatic groups having the structures (I) through (V): 
and Q is a linear or branched chain alkyl, alkyloxy, alkyl amine, alkyl sulfide, alkylene, alkyleneoxy, alkylene amine, alkylene sulfide, aryl, aryloxy, or aryl sulfide species having up to about 100 atoms in the chain, which may contain saturated or unsaturated cyclic or heterocyclic substituents pendant from the chain or as part of the backbone in the chain, and in which any heteroatom present may or may not be directly attached to X;
or Q is a urethane having the structure: 
xe2x80x83in which each R2 independently is an alkyl, aryl, or arylalkyl group having 1 to 18 carbon atoms; R3 is an alkyl or alkyloxy chain having up to 100 atoms in the chain, which chain may contain aryl substituents; X is O, S, N, or P; and n is 0 to 50;
or Q is an ester having the structure: 
xe2x80x83in which R3 is an alkyl or alkyloxy chain having up to 100 atoms in the chain, which chain may contain aryl substituents;
or Q is a siloxane having the structure:
xe2x80x94(CR12)exe2x80x94[SiR4xe2x80x94O]fxe2x80x94SiR42xe2x80x94(CR12)gxe2x80x94 in which the R1 substituent independently for each position is H or an alkyl group having 1 to 5 carbon atoms and the R4 substituent independently for each position is an alkyl group having 1 to 5 carbon atoms or an aryl group, and e and g are independently 1 to 10 and f is 1 to 50; and
m is 0 or 1, and n is 1 to 6.
Formula B represents those compounds in which
R9 is H, or an alkyl or alkyleneoxy group having 1 to 18 carbon atoms, or an allyl group, or an aryl or substituted aryl having the structure 
xe2x80x83in which R10, R11, and R12 are independently H or an alkyl or alkyleneoxy group having 1 to 18 carbon atoms;
Z is a linear or branched chain alkyl, alkyloxy, alkyl amine, alkyl sulfide, alkylene, alkyleneoxy, alkylene amine, alkylene sulfide, aryl, aryloxy, or aryl sulfide species having up to about 100 atoms in the chain, which may contain saturated or unsaturated cyclic or heterocyclic substituents pendant from the chain or as part of the backbone in the chain, and in which any heteroatom present may or may not be directly attached to K;
or Z is a urethane having the structure: 
xe2x80x83in which each R2 independently is an alkyl, aryl, or arylalkyl group having 1 to 18 carbon atoms; R3 is an alkyl or alkyloxy chain having up to 100 atoms in the chain, which chain may contain aryl substituents; X is O, S, N, or P; and n is 0 to 50;
or Z is an ester having the structure: 
xe2x80x83in which R3 is an alkyl or alkyloxy chain having up to 100 atoms in the chain, which chain may contain aryl substituents;
or Z is a siloxane having the structure:
xe2x80x94(CR12)exe2x80x94[SiR42xe2x80x94O]fxe2x80x94SiR42xe2x80x94(CR12)gxe2x80x94 in which the R1 substituent independently for each position is H or an alkyl group having 1 to 5 carbon atoms and the R4 substituent independently for each position is an alkyl group having 1 to 5 carbon atoms or an aryl group, and e and g are independently 1 to 10 and f is 1 to 50;
K is an aromatic group selected from the aromatic groups having the structures (VI) through (XIII) (although only one bond may be shown to represent connection to the aromatic group K, this will be deemed to represent any number of additional bonds as described and defined by n): 
xe2x80x83in which R5, R6, and R7 are a linear or branched chain alkyl, alkyloxy, alkyl amine, alkyl sulfide, alkylene, alkyleneoxy, alkylene amine, alkylene sulfide, aryl, aryloxy, or aryl sulfide species having up to about 100 atoms in the chain, which may contain saturated or unsaturated cyclic or heterocyclic substituents pendant from the chain or as part of the backbone in the chain, and in which any heteroatom present may or may not be directly attached to the aromatic ring; or R5, R6, and R7 are a siloxane having the structure xe2x80x94(CR12)exe2x80x94[SiR42xe2x80x94O]fxe2x80x94SiR42xe2x80x94(CH3)gxe2x80x94 in which the R1 substituent is H or an alkyl group having 1 to 5 carbon atoms and the R4 substituent independently for each position is an alkyl group having 1 to 5 carbon atoms or an aryl group, and e is 1 to 10 and f is 0 to 50; 
xe2x80x83and m is 0 or 1 and n is 1 to 6.
The compounds suitable for use in the adhesive compositions of this invention have a structure represented by one of the formulae:
[Mxe2x80x94Arm]nxe2x80x94Q or [Mxe2x80x94Zm]nxe2x80x94K, in which m is 0 or 1, and n is 1 to 6.
M represents a vinyl group and can be the maleimide moiety having the structure: 
xe2x80x83in which R1 is H or C1 to C5 alkyl; or or the vinyl moiety having the structure: 
xe2x80x83in which R1 and R2 are H or an alkyl having 1 to 5 carbon atoms, or together form a 5 to 9 membered ring with the carbons forming the vinyl group; B is C, S, N, O, C(O), Oxe2x80x94C(O), C(O)xe2x80x94O, C(O)NH or C(O)N(R8), in which R8 is C1 to C5 alkyl. Preferably, B is O, C(O), Oxe2x80x94C(O), C(O)xe2x80x94O, C(O)NH or C(O)N(R8); more preferably B is O, C(O), Oxe2x80x94C(O), C(O)xe2x80x94O, or C(O)N(R8).
Ar independently is an aromatic group selected from the aromatic groups having the structures (I) through (V): 
Preferably, Ar is structure (II), (III), (IV) or (V), and more preferably is structure (II).
Q and Z independently can be a linear or branched chain alkyl, alkyloxy, alkyl amine, alkyl sulfide, alkylene, alkyleneoxy, alkylene amine, alkylene sulfide, aryl, aryloxy, or aryl sulfide species having up to about 100 atoms in the chain, which may contain saturated or unsaturated cyclic or heterocyclic substituents pendant from the chain or as part of the backbone in the chain, and in which any heteroatom present may or may not be directly attached to X;
or Q and Z independently can be a urethane having the structure: 
xe2x80x83in which each R2 independently is an alkyl, aryl, or arylalkyl group having 1 to 18 carbon atoms; R3 is an alkyl or alkyloxy chain having up to 100 atoms in the chain, which chain may contain aryl substituents; X is O, S, N, or P; and n is 0 to 50;
or Q and Z independently can be an ester having the structure: 
xe2x80x83in which R3 is an alkyl or alkyloxy chain having up to 100 atoms in the chain, which chain may contain aryl substituents;
or Q and Z independently can be a siloxane having the structure: xe2x80x94(CR12)exe2x80x94[SiR4xe2x80x94O]fxe2x80x94SiR42xe2x80x94(CR12)gxe2x80x94 in which the R1 substituent independently for each position is H or an alkyl group having 1 to 5 carbon atoms and the R4 substituent independently for each position is an alkyl group having 1 to 5 carbon atoms or an aryl group, and e and g are independently 1 to 10 and f is 1 to 50.
Preferably, Q and Z will be a linear or branched chain alkyl, alkyloxy, alkylene, or alkyleneoxy species having up to about 100 atoms in the chain, as described with pendant saturated or unsaturated cyclic or heterocyclic substituents, or a siloxane as described, and more preferably is a linear or branched chain alkyl species or siloxane, as described.
K is an aromatic group selected from the aromatic groups having the structures (VI) through (XIII) (although only one bond may be shown to represent connection to the aromatic group K, this will be deemed to represent any number of additional bonds as described and defined by n): 
in which R5, R6, and R7 are a linear or branched chain alkyl, alkyloxy, alkyl amine, alkyl sulfide, alkylene, alkyleneoxy, alkylene amine, alkylene sulfide, aryl, aryloxy, or aryl sulfide species having up to about 100 atoms in the chain, which may contain saturated or unsaturated cyclic or heterocyclic substituents pendant from the chain or as part of the backbone in the chain, and in which any heteroatom present may or may not be directly attached to the aromatic ring or R5, R6, and R7 are a siloxane having the structure xe2x80x94(CR12)exe2x80x94[SiR42xe2x80x94O]fxe2x80x94SiR42xe2x80x94(CH3)gxe2x80x94 in which the R1 substituent is H or an alkyl group having 1 to 5 carbon atoms and the R4 substituent independently for each position is an alkyl group having 1 to 5 carbon atoms or an aryl group, and e is 1 to 10 and f is 0 to 50; 
Preferably, K is structure (VIII), (X) or (XI), more preferably is structure (X) or (XI), and most preferably is structure (X).
Depending on the nature of the substrate, the composition may also contain a coupling agent. A coupling agent as used herein is a chemical species containing a polymerizable functional group for reaction with the maleimide and other vinyl compound, and a functional group capable of condensing with metal hydroxides present on the surface of the substrate. Such coupling agents and the preferred amounts for use in compositions for particular substrates are known in the art. Suitable coupling agents are silanes, silicate esters, metal acrylates or methacrylates, titanates, and compounds containing a chelating ligand, such as phosphine, mercaptan, and acetoacetate. When present, coupling agents typically will be in amounts up to 10 percent by weight, and preferably in amounts of 0.1-3.0 percent by weight, of the maleimide and other monofunctional vinyl compound.
In addition, the compositions may contain compounds that lend additional flexibility and toughness to the resultant cured composition. Such compounds may be any thermoset or thermoplastic material having a Tg of 50xc2x0 C. or less, and typically will be a polymeric material characterized by free rotation about the chemical bonds, such as can be obtained by the presence of carbon-carbon double bonds adjacent to carbon-carbon single bonds, the presence of ester and ether groups, and the absence of ring structures. Suitable such modifiers include polyacrylates, poly(butadiene), polyTHF (polymerized tetrahydrofuran), CTBN (carboxy-terminated butyronitrile) rubber, and polypropylene glycol. When present, toughening compounds may be in an amount up to about 15 percent by weight of the maleimide and other monofunctional vinyl compound.
Siloxanes may also be added to the compositions to impart elastomeric properties. Suitable siloxanes are the methacryloxypropyl-terminated polydimethyl siloxanes, and the aminopropyl-terminated polydimethylsiloxanes, available from United Chemical Technologies.
The composition may also contain organic fillers, such as, polymers to adjust rheology. Other additives known and used in the art may also be used for specific purposes, such as, adhesion promoters. The selection of the types and amounts suitable is within the expertise of one skilled in the art.